The invention relates to laser optics and to a diode laser therewith.
In contrast to conventional laser beam sources which have a beam diameter of a few mm with low beam divergence in the range of a few mrad, the radiation of a semiconductor diode laser (hereinafter also "diode laser") is characterized by a highly divergent beam with a divergence &gt;1000 mrad. This is caused by the exit layer which is limited to &lt;1 micron in height, on which, like diffraction on a gap-shaped opening, a greater divergence angle is produced. Since the extension of the exit opening in the plane is different perpendicular and parallel to the active semiconductor layer, various beam divergences occur in the plane perpendicular and parallel to the active layer.
To achieve a power from 20 to 40 W for a diode laser, numerous laser chips are combined on a so-called bar to form a laser component. Conventionally 10-50 individual emitter groups are arranged in a row in the plane parallel to the active layer. The resulting beam of this bar in the plane parallel to the active angle has an opening angle of roughly 10.degree. and a beam diameter of roughly 10 mm. The resulting beam quality in this plane is several times less than the resulting beam quality in the above described plane perpendicular to the active layer. In a possible further reduction of the divergence angle of laser chips the ratio of the beam quality perpendicular and parallel to the active layer differs greatly.
The beam has a major difference of beam quality in the two directions perpendicular and parallel to the active layer based on the above described beam characteristic. The concept of beam quality is described by the M.sup.2 parameter. M.sup.2 is defined by the factor with which the beam divergence of the diode laser beam exceeds the beam divergence of a diffraction-limited beam of the same diameter. In the aforementioned case, in the plane parallel to the active layer there is a beam diameter which exceeds the beam diameter in the vertical plane by a factor of 10 000. In beam divergence the behavior is somewhat different, i.e. in the plane parallel to the active layer beam divergence is less by almost a factor of 10. The M.sup.2 parameter in the plane parallel to the active layer is thus several orders of magnitude above the M.sup.2 value in the plane perpendicular to the active layer.
One possible objective of beam shaping is to achieve a beam with almost the same M.sup.2 values in both planes, i.e. perpendicular and parallel to the active later. Currently the following methods are known for shaping the beam geometry, by which similarity of beam qualities in the two main planes of the beam is achieved.
By means of a fiber bundle linear beam cross sections can be combined to form a circular bundle by rearranging the fibers. These processes are described for example in U.S. Pat. Nos. 5,127,068; 4,763,975; 4,818,062; 5,268,978; and 5,258,989.
In addition, there is the technique of beam rotation in which the radiation of individual emitters is turned by 90.degree. in order to undertake rearrangement, in which an arrangement of the beams in the direction of the axis of better beam quality takes place. The following arrangements for this process are known:
U.S. Pat. No. 5,168,401; EP 0 484 276; and DE 4 438 368. It is common to all processes that the radiation of a diode laser after its collimation in the fast axis direction is turned by 90.degree. in order to effect slow axis collimation with common cylinder optics. In one modification of this process a continuous line source is also conceivable (for example, that of a diode laser of high occupation density which is collimated in the fast axis direction) with a beam profile (line) which is divided and which is present in rearranged form behind the optical element.
In addition, it is possible to undertake rearrangement of the radiation of individual emitters without rotating the beam, for example by parallel offset (shifting) by means of parallel mirrors rearrangement of the radiation being achieved (WO 95/15510). An arrangement which likewise uses the technique of rearrangement is described in DE 195 00 53 and DE 19 5 44 488. In this case the radiation of a diode laser bar is deflected into different planes and is individually collimated there.
The disadvantages of the prior art can be summarized among others in that for fiber-coupled diode lasers generally a beam with very different beam qualities in the two axial directions is coupled into the fiber. For a round fiber this means that in one axial direction the possible numerical aperture or fiber diameter is not used. This leads to major losses in power density so that in practice there is a limitation to roughly 10.sup.4 W/cm.sup.2.
In these known processes, to some extent major path length differences must be furthermore compensated. This is generally done by correction prisms which can compensate for errors only to a limited degree. Multiple reflections furthermore impose increased demands for adjustment precision, production tolerances and component stability (WO 95/15510). Reflecting optics (for example, of copper) have high absorption values.
The object of the invention is to devise laser optics which avoid the aforementioned disadvantages, and with the possibility of simple and economical production enables reshaping of the laser beam in the desired manner.